attoparsec

About 31 mm (one inch). "atto-" is the
standard SIprefix for multiplication by 10^-18. A parsec
(parallax-second) is 3.26 light-years; an attoparsec is thus
3.26 * 10^-18 light years. Thus, one attoparsec per
microfortnight is about one inch per second.

This unit is reported to be in use (though probably not very
seriously) among hackers in the UK.